1. Field of the Invention
The present invention relates to a metal gasket, and more particularly to a metal gasket disposed between a cylinder head and a cylinder block of an internal combustion engine, and formed of a metallic base plate defining a plurality of openings and having beads formed therearound.
2. Description of the Related Art
For sealing a clearance between the surfaces to be sealed, e.g., a cylinder head and a cylinder block of an internal combustion engine, various gaskets are disposed therebetween. The gasket for use of the internal combustion engine defines a plurality of openings, e.g., combustion openings, fluid openings such as coolant openings and oil openings, and bolt holes, and is clamped between the cylinder head and cylinder block so that combustion gas, coolant and oil will not spread from those openings. In order to provide a uniform sealing between the cylinder head and cylinder block, it has been proposed to employ a metal gasket, e.g., an elastic metallic base plate defining those openings and having beads formed therearound to ensure a high sealing pressure especially around the combustion openings. The beads may be formed along the periphery of the base plate, or may be formed around both fluid openings and bolt holes as disclosed in Japanese Utility model Laid-Open Publication No.62-86459 and No.59-188351 respectively.
In those prior metal gaskets, it has been proposed to provide a resilient sealing member on the base plate along a hem of each bead to thereby improve a sealing property around the combustion opening, as disclosed in Japanese Utility model Laid-Open Publication No.64-8556 for example, or to provide the sealing member deposited in recesses of the beads, as disclosed in Japanese Utility model Laid-Open Publication No.60-18247. Also, there is disclosed a cylinder head metal gasket having a heat resistant elastic material layer formed around a part of the bead or its whole periphery, in Japanese Utility model Laid-Open Publication 1-141354. However, it may not necessarily provide a sufficient sealing property around the coolant openings which are provided around the combustion openings, so that a certain space is made between those openings. As a result, coolant may spread to reach a periphery of the base plate staining a surface of the cylinder head.
When the cylinder head and cylinder block are clamped together, a clearance therebetween at the side of the combustion opening of the gasket becomes larger than that at the side of the bolt holes. Therefore, while it is desirable to provide a sealing member around coolant openings, it is not appropriate by simply surrounding a sealing member of an equal sectional dimension around the coolant opening.
Further, in Japanese Utility model Laid-Open Publication No.59-188955, there is disclosed a plate for adjusting a sealing pressure whose thickness is smaller than the height of the bead, and which is disposed adjacent to the bead, so as to avoid a reduction in sealing pressure due to undesirable plastic deformation of the bead caused by a clamping force applied thereto. Likewise, a thick member is proposed to be disposed on the base plate adjacent to the bead. With respect to this thick member, there is proposed a thick member formed by metal plating in Japanese Utility model Laid-Open Publication No.61-14748, an auxiliary plate adhered or welded on the base plate in Japanese Utility model Laid-Open Publication No.2-6855, and a coated stopper (corresponding to the thick member) with plastic or the like impregnated therein for providing a sealing effect in Japanese Patent Laid-Open Publication No.64-35057. Japanese Patent Laid-Open Publication No.62-278375, discloses a gasket of a plate defining more than three combustion openings in series and having annular shims disposed to surround respective combustion openings.
In order to prevent the bead from impairing the surface to be sealed, it is proposed, in Japanese Utility model Laid-Open Publication No.61-41960, to deposit on the base plate a first vulcanized coating layer and further deposit a second soft non-vulcanized coating layer to compensate a rough surface of the base plate. Also, it is proposed, in Japanese Utility model Laid-Open Publication No.64-8556, to coat all the surfaces of the base plate with microseal material to protect the bead or the like.
However, in all of these metal gaskets, it is difficult to provide effective sealing, especially to ensure a substantially uniform sealing pressure around the openings such as fluid openings, in accordance with the gaskets in the clamped condition thereof between the cylinder head and cylinder block. Thus, it would be desirable to provide a metal gasket adapted for an internal combustion engine with more effective sealing.